Anchoring rating scale structures

Anchoring (fixing) the Rasch-Andrich thresholds (step calibrations) of a rating-scale (or partial-credit scale) enables rating scales in one analysis to be equated to rating scales in another analysis.

Anchoring also enables predetermined threshold values to be imposed on an analysis.

5 = bottom, 0, A ; the bottom category has the dummy anchor-value of 0.

6 = , -7.59, A

.....

12 = , 8.94, A

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Pivot points

The Rating scale= specification provides the capability to choose the pivot point in a dichotomy, rating scale, partial credit or other polytomous structure.

Example: I am setting the standard for a "Competent" essay-writer. I need to anchor the mean for the essays at a particular point on the scale - "good" (6) and/or halfway between 5 and 6. How do I do this?

Here is Table 8 from an unanchored run of Essays.txt. Looking at the "Rasch-Andrich Thresholds" (step calibrations), the standard 0 logit point for the scale is the point on the latent variable at which categories 1 and 9 (the lowest and highest categories) are equally probable.

Example 2: I want to set the person-item targeting on dichotomous items at 2 logits offset (= 88% probability of success) instead of the usual 0 logits offset (=50% chance of success). This will make all the persons appear to be 2 logits less able.